As is well known, the process control systems of compressors are designed for changing their performance to fit the requirements of the user's process, e.g., to control discharge pressure or flow rate. On the other hand, in order to protect the compressor from approaching the surge zone, an antisurge protective system must limit the possible range of safe operation by a surge control line. If an operating point of the compressor having the conventional control and protective loops reaches the surge control line, then a counteraction between the process control loop and antisurge protective loop appears. The process control loop can continue to move the operating point towards the surge zone and this interferes with the protective system designed to keep the compressor from approaching the surge zone, especially when the protective loop controlling the relief valve includes one or more elements having nonlinearities, e.g., hysteresis or dead zone.
The deviation of compressor operating point from the surge control line depends on the speed of the disturbance and the magnitude of dead zone or hysteresis. It is well known that conventional antisurge protective systems are not able to prevent the operating point of a compressor from crossing its surge limit line if the disturbances are very big or very fast and are not able to stop surge if it appears. However, the same conventional systems may stop the operating point of a compressor on the surge control line, without dangerous deviation, only under slow disturbances. The operation of such antisurge protective system may be considerably improved during fast disturbances by a fast opening of the relief valve, when a preestablished deviation of the operating point of the compressor from the surge control line appears, and then by slow closing of said valve. This may also stop surge at the very beginning if the location of surge control line is changed at the same time.